Mattress spring core construction over the years has been a continuously improving art with advancements in materials and machine technology. A well-known form of spring core construction is known as a Marshall spring construction wherein metal coil springs are encapsulated in individual pockets of fabric and formed as elongate or continuous strings of pocketed coil springs. In an earlier form, these strings of coil springs were manufactured by folding an elongate piece of fabric in half lengthwise to form two plies of fabric and stitching transverse and longitudinal seams to join the plies of fabric to define pockets within which the springs were enveloped.
More recently, improvements in spring core constructions have involved the use of fabrics which are thermally or ultrasonically weldable to themselves. By using such welding techniques, these fabrics have been advantageously used to create strings of individually pocketed coil springs wherein transverse and longitudinal welds, instead of stitching, are used to form the pockets encapsulating the springs. A fabric which has been used and proven to ultrasonically weld to itself is a non-woven polypropylene fabric.
Due to its low cost, a single-layer of non-woven spun-bonded polypropylene fabric is the most common fabric used in the manufacture of strings of springs used in pocketed spring assemblies. The weight of such a single-layer of non-woven spun-bonded polypropylene fabric is commonly between 1-3 ounces per square yard. While single layer non-woven spun-bonded polypropylene fabric has been attractive to many pocketed spring core manufacturers because of its accessibility and low cost, such fabric has one drawback.
A pocketed spring assembly comprising strings of individually pocketed springs made from such single layer non-woven spun-bonded polypropylene fabric may create “noise”, as known in the industry. Such noise may be created by the fabric expanding upon removal of the load due to the coil spring's upwardly directed force on the fabric. This noise may be described as a “rustling” of the fabric as the fabric rubs on adjacent metal coil springs or on other upholstery materials such as urethane foam, latex or fiber.
Noise, as it is known in the industry, may be reduced by reducing the weight of the single-layer non-woven spun-bonded polypropylene fabric. However, such lighter weight single-layer non-woven spun-bonded polypropylene fabric has proven to be insufficient for processing the fabric through a coiler and properly retaining the metal coil springs in their respective pockets.
It is therefore an objective of this invention to provide a pocketed spring assembly made, at least partially, with dual layer, non-woven spun-bonded polypropylene fabric which has adequate strength for processing the fabric during manufacture of strings of springs but results in reduced noise.
It is further an objective of this invention to provide a pocketed spring assembly made, at least partially, with dual layer, non-woven spun-bonded polypropylene fabric strong enough hold up during the manufacturing process but which reduces the noise when incorporated into the pocketed spring assembly.
It is therefore an objective of this invention to provide a dual or double-layer non-woven spun-bonded polypropylene fabric for a pocketed spring assembly which has adequate strength to withstand the process of being used in a pocket coiler, but which results in less noise than a conventional single-layer non-woven spun-bonded polypropylene fabric when incorporated into a pocketed spring assembly.
It is another objective of this invention to provide a method of making a double-layer non-woven spun-bonded polypropylene fabric for a pocketed spring assembly which ultrasonically welds to itself, has improved strength when compared to a single-layer non-woven spun-bonded polypropylene fabric, and is quiet.